Polymeric materials are used in a variety of optical products due to their being lightweight and inexpensive to produce. Polycarbonates, for example, are characterized by excellent clarity, resistance to discoloration, high strength and high impact resistance.
Most polymers are synthesized by either a thermal or photocuring process. Variations in these curing processes may influence the performance of the prepared polymer. For example, thermal polymerization may be accompanied by high shrinkage during cure, for example, in the range of from about 10 to about 20%, and extended curing times, for example, in the range of from about 5 to about 16 hours or more. The high shrinkage levels create difficulties in the production of precision optics such as lenses or prisms, particularly in the production of articles having larger thicknesses or large differences in thickness between the center and the edges of the article. The extended cure times tie up production facilities, and may lead to inefficient utilization of the dies in which the articles are molded. Also, the thermal cure cycle used to polymerize the monomer consumes large amounts of energy, and may thermally stresses the dies.
With respect to the photocuring process, polymeric reactions may be initiated by irradiation at a wavelength of about 780 nm or less, especially, in the range of the ultraviolet (UV) spectra, for example, in the range of from about 10 to about 380 nm. These reactions progress rapidly to form numerous chains between the monomers and/or oligomers. Owing to characteristics of this reaction, it may not be possible to determine the precise molecular weight of the polymer obtained by photocuring, as well as the glass transition temperature (Tg). However, increasing the molecular weight of a polymeric material generally improves the density, and may thereby enhances the toughness and hardness of the material.
High molecular weight polymers may be formed into optical products by either an injection or extrusion process. Optical products prepared by these processes however may have certain disadvantages such as, for example, shrinkage after polymerization; cracking caused by temperature and/or moisture variations; expansion due to moisture absorption resulting in reduced birefringence, and so forth. As an increasing number of polymeric electronic products are manufactured employing high intensity light, the heat (with a temperature in the range of from about 60 to about 160° C.) to which those parts directly in contact with the light source are subjected may cause problems such as reduced dimensional stability and surface aging. Also, optical products that are stored and operated under conditions of high temperature and high humidity (for example more than about 40° C. and 75% RH) experience a shortened lifespan. Thus, there remains a need in the art for improved polymeric materials for use in optical products.